There is a growing need for multiband basestation antennas for mobile communication systems, to serve existing 2.sup.nd generation systems, and emerging third generation systems. For example, GSM and DCS1800 systems currently coexist in Europe, and emerging 3.sup.rd generation systems (UMTS) will initially have to operate in parallel with these systems. At a given base site there may be a need to cover all three bands, and if separate antennas are used for each band this results in an unacceptably large number of antennas. Typically, two antennas are used per sector at a base site, which allows for receive diversity on the uplink. Consequently, for a base site covering all three bands this would result in 6 antennas for an omnidirectional base site, and 18 antennas for a trisector, or tricellular arrangement. The problem is similar in North America where AMPS/NADC, PCS, and 3.sup.rd generation systems will have to coexist.
Some of the frequency bands of interest are shown in Tables 1-3. Table 1 shows the frequency bands of some first and second generation systems. Table 2 shows the IMT-2000 recommendations regarding frequency allocations for third generation systems, along with the actual spectrum availability in Europe. Table 3 shows the spectrum availability in various parts of the world compared to the IMT-2000 recommendations.
There are a number of issues to consider regarding the basestation antenna. Firstly, it would be preferred that a single structure covering all three frequency bands exists to minimise the number of antennas at any given base site. It would be preferred that the different bands should therefore have a shared aperture. The antenna structure should be designed for ease-of-manufacture and it should also be designed such that the structure has minimum cost. It is possible that antennas of different beamwidths will be required for different cell types (eg. Omni-, trisectored, tricellular, microcell) and so the design should be flexible enough to allow for this. In addition, the number of antennas can be minimised if polarisation diversity is employed rather than space diversity, such that dual polarised antenna configurations need to be considered.
Some cellular basestation antenna manufacturers have dual frequency band dual polar products, but these comprise colocated separate antennas, the separate antennas being used for the two separate bands and are simply stacked on top of each other, the antennas having been packaged as a single item or placed side by side. Vertically polarised antennas are known for use in the UMTS 1920-2170 MHz range, but commercial versions of DCS1800/UMTS cross polar antennas have yet to appear on the market. Large structures, however, are not favoured by town planners and the like: base station structures should be as small and as inconspicuous as possible.
Basestation antennas are generally array antennas, since these allow flexibility in the control of the radiation pattern. The pattern characteristics can be varied by altering the individual element amplitude and phase weights, which is useful for providing electrical downtilt, and for providing null fill-in. However, arrays are inherently narrowband because the electrical separation distance between elements changes with frequency, and this affects the array performance. In particular, if the element separation becomes too large (electrically) then grating lobes will appear in the pattern, where these are secondary main lobes. These cause a reduction in gain and an increase in the interference in the network (if they appear in the azimuth plane).
Due to the narrowband characteristics of array antennas, the use of wideband arrays has been very restricted. In the design of a wideband array, the wideband properties of the individual elements, and the wideband characteristics of the array must be considered separately.
In `The Three-Dimensional Frequency-independent Phased Array (3D-FIPA)`, J. K. Breakall, IEE Ninth International Conference on Antennas and Propagation, ICAP '95, Conference publication No. 407, pp.9-11 a design is presented for a three-dimensional frequency-independent phased array (3D-FIPA) which at the IEE ICAP '95 conference. This is achieved by applying a log-periodic principle whereby multilayer dipole arrays are formed that maintain all electrical spacings and heights over a user specified range. The design results in an antenna that maintains nearly constant pattern characteristics, gain, and VSWR over a wide bandwidth. FIG. 1 shows top and side views of the form of the array where dual polar elements (crossed dipoles) are employed. The uppermost layer of dipoles are shown emboldened to illustrate the layer that would be excited at the lowest frequency of operation.
The 3D-FIPA preserves all spacings and heights above ground (expressed in wavelengths) for active elements as the frequency is varied. However, the ground plane size does not scale with frequency but has a fixed physical size. This will introduce a frequency dependent effect on the antenna performance. In view of the three dimensional nature of the array it may become difficult to manufacture a low cost structure if many dipole layers are required.
In Wideband Arrays with variable element sizes', D. G. Shively, W. L. Stutzman, IEE Proc., Vol.137, Pt. H, No.4, August 1990 a wideband array structure is presented that operates over a two octave bandwidth. The array consists of large and small cavity-backed Archimedean spiral elements in alternate positions. The general planar case is a filled grid version of the array shown in FIG. 1b.
The diameter of the large spirals is twice that of the small spirals. These elements are circularly polarised and radiate when the perimeter of the spiral is approximately one wavelength. Consequently, the maximum spiral perimeter (dictated by the diameter) determines the lowest frequency of operation. As the frequency is increased, the location of the active region of the spiral moves towards the centre of the spiral. However, the aperture size does not scale with frequency, and consequently, the gain and beamwidth of the array do not remain constant with frequency. In fact, the gain increases with frequency as the beamwidth decreases and therefore is not suitable for a multiband basestation antenna.